There exist numerous AC switching applications, such as power controllers, uninterruptable power supplies, and motor controls, that require:                1. switching an AC load on and off; and        2. that benefit significantly by simultaneously monitoring electrical current flowing through the AC load.        
For most AC monitoring applications, the monitoring circuit must be electrically isolated from the AC power supplied to the load. Conventional electrically isolated AC monitoring circuits generally include either:                1. an optical isolator consisting of a photodiode combined with a LED connected in series with the AC power source and the load;        2. a current sensing transformer having a winding that is wired in series with the AC power source and the load;        3. a hall effect sensor that includes a magnetic field detector located nearby a high current conductor connected in series with the AC power source and the load; or        4. a high impedance differential amplifier which provides a high degree of virtual isolation through the use of large value series resistors connected to both sides of a resistive shunt connected in series with the AC power source and the load.        
Processing a digitizing AC power signal to determine the phase of the AC power signal is well known. For example, U.S. Pat. No. 4,077,061 entitled “Digital Processing and Calculating AC Electric Energy Metering System” discloses a metering system for electric utility power line AC power measurements that digitizes randomly sampled AC voltage and current signals, and computes therefrom electric energy parameters. Similarly, U.S. Pat. No. 5,260,647 entitled “Measuring an AC Signal Value with Sampling When the Sampling Interval Does Not Exactly Divide the AC Signal's Period” discloses computer software that processes a digitally sampled AC waveform to accurately measure a sampled AC voltage. The disclosed computer software accurately measures a sampled AC voltage:                1. even though the available sampling intervals do not exactly divide the period of the AC signal to be measured; and        2. without averaging over a plurality of periods which is exactly divisible (or nearly so) by the sampling interval.        
Also, U.S. Pat. No. 5,528,134 entitled “AC Power Analyzer” discloses computer software that performs spectrum analysis on the resampled points of an AC power waveform to reduce to zero artifacts at the harmonic intervals of the AC power frequency.
An electrically isolated relay circuit for switching AC power to a load on and off that also permits sensing electrical current flowing through the load while avoiding use of an optical isolator, a current sensing transformer or an operational amplifier with large value resistors would be economically advantageous. For many applications, an isolated AC switching and sensing circuit that requires fewer parts increases the practicality of sensing AC power, particularly for those applications in which the complexity or cost of a separate electrically isolated current sensor would otherwise be too expensive. The economic advantage of being able to sense electrical current flowing through a load while avoiding an additional optical isolator, transformer or operational amplifier increases even more if the current sensing components can be timeshared among several different loads.